English

Noun

A microorganism (also can be spelled as micro
organism) or microbe is an organism that is microscopic (too small to be
seen by the naked human eye). The study of microorganisms is called
microbiology, a
subject that began with Anton
van Leeuwenhoek's discovery of microorganisms in 1675, using a
microscope of his own
design.

Microorganisms are incredibly diverse and include
bacteria, fungi, archaea, and protists, as well as some
microscopic plants and
animals such
as plankton, and
popularly-known animals such as the planarian and the amoeba. They do not include
viruses and prions, which are generally
classified as non-living. Most microorganisms are single-celled, or
unicellular, but some multicellular organisms are microscopic,
while some unicellular protists, and a bacteria called Thiomargarita
namibiensis are visible to the naked eye.

History

Evolution

further Timeline
of evolution Single-celled microorganisms were the first forms
of life to develop on earth, approximately 3–4 billion years
ago. Further evolution was slow, and for about 3 billion years
in the Precambrianeon, all
organisms were microscopic. So, for most of the history of life on
Earth the only form of life were microorganisms. Bacteria,
algae and fungi have been identified in amber that is 220 million years
old, which shows that the morphology of microorganisms has changed
little since the triassic period.

Pre-Microbiology

The possibility that microorganisms might
exist was discussed for many centuries before their actual
discovery in the 17th century. The first ideas about microorganisms
were those of the RomanscholarMarcus
Terentius Varro in a book titled On Agriculture in which he
warns against locating a homestead near swamps: This passage seems
to indicate that the ancients were aware of the possibility that
diseases could be spread by yet unseen organisms.

When the Black Deathbubonic
plague reached al-Andalus in
the 14th century, Ibn Khatima wrote that infectious diseases were
caused by "contagious entities" that enter the human body. Later,
in 1546, Girolamo
Fracastoro proposed that epidemicdiseases were caused by
transferable seedlike entities that could transmit infection by
direct or indirect contact, or even without contact over long
distances.

All these early claims about the existence of
microorganisms were speculative in nature and
not based on any data or science. Microorganisms were neither
proven, observed, nor correctly and accurately described until the
17th century. The reason for this was that all these early
inquiries lacked the most fundamental tool in order for microbiology and bacteriology to exist as a
science, and that was the microscope.

Discovery

Anton
van Leeuwenhoek was the first person to observe microorganisms,
using a microscope of
his own design, thereby making him the first microbiologist. In doing
so Leeuwenhoek would make one of the most important contributions
to biology and open up
the fields of microbiology and bacteriology. Prior to
Leeuwenhoek's discovery of microorganisms in 1675, it had been a
mystery as to why grapes
could be turned into wine,
milk into cheese, or why food would spoil.
Leeuwenhoek did not make the connection between these processes and
microorganisms, but using a microscope, he did establish that there
were forms of life that were not visible to the naked eye.
Leeuwenhoek's discovery, along with subsequent observations by
Lazzaro
Spallanzani and Louis
Pasteur, ended the long-held belief that life spontaneously
appeared from non-living substances during the process of
spoilage.

Lazzarro Spallanzani found that microorganisms
could only settle in a broth if the broth was exposed to the air.
He also found that boiling the broth would sterilise
it and kill the microorganisms. Louis Pasteur expanded upon
Spallanzani's findings by exposing boiled broths to the air, in
vessels that contained a filter to prevent all particles from
passing through to the growth medium, and also in vessels with no
filter at all, with air being admitted via a curved tube that would
not allow dust particles to come in contact with the broth. By
boiling the broth beforehand, Pasteur ensured that no
microorganisms survived within the broths at the beginning of his
experiment. Nothing grew in the broths in the course of Pasteur's
experiment. This meant that the living organisms that grew in such
broths came from outside, as spores on dust, rather than
spontaneously generated within the broth. Thus, Pasteur dealt the
death blow to the theory of spontaneous generation and supported
germ
theory.

In 1876, Robert Koch
established that microbes can cause disease. He did this by finding
that the blood of cattle who were infected with anthrax always had large numbers
of Bacillus
anthracis. Koch also found that he could transmit anthrax from
one animal to another by taking a small sample of blood from the
infected animal and injecting it into a healthy one, causing the
healthy animal to become sick. He also found that he could grow the
bacteria in a nutrient broth, inject it into a healthy animal, and
cause illness. Based upon these experiments, he devised criteria
for establishing a causal link between a microbe and a disease in
what are now known as Koch's
postulates. Though these postulates cannot be applied in all
cases, they do retain historical importance in the development of
scientific thought and can still be used today.

Classification and structure

Microorganisms can be found
almost anywhere in the taxonomic organization of life
on the planet. Bacteria and
archaea are almost
always microscopic, while a number of eukaryotes are also
microscopic, including most protists, some
fungi, as well as some
animals and
plants. Viruses are
generally regarded as not living and therefore are not microbes,
although the field of microbiology also
encompasses the study of viruses.

Prokaryotes

Prokaryotes are organisms that lack a cell nucleus
and the other organelles found in eukaryotes. Prokaryotes are
almost always unicellular, although some species such as myxobacteria can aggregate
into complex structures as part of their life cycle.
These organisms are divided into two groups, the archaea and the
bacteria.

Bacteria

Bacteria are the
most diverse and abundant group of organisms on Earth. Bacteria
inhabit practically all environments where some liquid water is
available and the temperature is below +140 °C. They are found in
sea
water, soil, air,
animals' gastrointestinal
tracts, hot springs
and even deep beneath the Earth's crust in rocks.
Practically all surfaces which have not been specially sterilized
are covered in bacteria. The number of bacteria in the world is
estimated to be around five million trillion trillion, or 5
× 1030.

Bacteria are practically all invisible to the
naked eye, with a few extremely rare exceptions, such as Thiomargarita
namibiensis. They are unicellular organisms and
lack membrane-bound organelles. Their genome is usually a single
loop of DNA,
although they can also harbor small pieces of DNA called plasmids. These plasmids can be
transferred between cells through bacterial
conjugation. Bacteria are surrounded by a cell wall,
which provides strength and rigidity to their cells. They reproduce
by binary
fission or sometimes by budding, but do not undergo
sexual
reproduction. Some species form extraordinarily resilient
spores, but for
bacteria this is a
mechanism for survival, not reproduction. Under optimal conditions
bacteria can grow extremely rapidly and can double as quickly as
every 10 minutes.

Archaea

Archaea are also single-celled organisms that lack
nuclei. In the past, the differences between bacteria and archaea
were not recognised and archaea were classified with bacteria as
part of the kingdom Monera. However, in
1990 the microbiologist Carl Woese
proposed the three-domain
system that divided living things into bacteria, archaea and
eukaryotes. Archaea differ from bacteria in both their genetics and
biochemistry. For example, while bacterial cell
membranes are made from phosphoglycerides with
ester bonds, archaean
membranes are made of ether
lipids.

Archaea were originally described in extreme
environments, such as hot springs,
but have since been found in all types of habitats. Only now are
scientists beginning to appreciate how common archaea are in the
environment, with crenarchaeota being the
most common form of life in the ocean, dominating ecosystems below
150 m in depth. These organisms are also common in soil and play a
vital role in ammonia
oxidation.

Animals

All animals are multicellular, but some are too
small to be seen by the naked eye. Microscopic arthropods include dust mites and
spider
mites. Microscopic crustaceans include copepods and the cladocera, while many nematodes are too small to be
seen with the naked eye. Another particularly common group of
microscopic animals are the rotifers, which are filter
feeders that are usually found in fresh water. Micro-animals
reproduce both sexually and asexually and may reach new habitats as
eggs that survive harsh environments that would kill the adult
animal. However, some simple animals, such as rotifers and
nematodes, can dry out completely and remain dormant for long
periods of time.

Plants

The green algae
are a large group of photosynthetic eukaryotes that include many
microscopic organisms. Although some green algae are classified as
protists, others such as
charophyta are
classified with embryophyte plants, which
are the most familiar group of land plants. Algae can grow as
single cells, or in long chains of cells. The green algae include
unicellular and colonial flagellates, usually but not
always with two flagella per cell, as well as
various colonial, coccoid, and filamentous forms.
In the Charales, which
are the algae most closely related to higher plants, cells
differentiate into several distinct tissues within the organism.
There are about 6000 species of green algae.

Habitats and ecology

Microorganisms are found in almost
every habitat
present in nature. Even in hostile environments such as the
poles,
deserts, geysers, rocks, and
the deep
sea, some types of microorganisms have adapted to the extreme
conditions and sustained colonies; these organisms are known as
extremophiles.
Extremophiles have been isolated from rocks as much as 7 kilometres
below the earth's surface, and it has been suggested that the
amount of living organisms below the earth's surface may be
comparable with the amount of life on or above the surface. Many
types of microorganisms have intimate symbiotic relationships with
other larger organisms; some of which are mutually beneficial
(mutualism), while
others can be damaging to the host
organism (parasitism). If
microorganisms can cause disease in a host they are known
as pathogens.

Extremophiles

Extremophiles
are microorganisms which have adapted so that they can survive and
even thrive in conditions that are normally fatal to most
lifeforms. For example, some species have been found in the
following extreme environments:

Extremophiles are significant in different ways.
They extend terrestrial life into much of the Earth's hydrosphere, crust and
atmosphere, their specific evolutionary adaptation mechanisms to
their extreme environment can be exploited in bio-technology,
and their very existence under such extreme conditions increases
the potential for extraterrestrial
life.

Soil microbes

The nitrogen
cycle in soils depends on the fixation of atmospheric nitrogen.
One way this can occur is in the nodules in the roots of legumes that contain symbiotic
bacteria of the genera Rhizobium, Mesorhizobium, Sinorhizobium,
Bradyrhizobium, and Azorhizobium.

Symbiotic microbes

Symbiotic microbes

Importance

Microorganisms are vital to humans and the
environment, as they participate in the Earth's element cycles such
as the carbon cycle
and nitrogen
cycle, as well as fulfilling other vital roles in virtually all
ecosystems, such as
recycling other organisms' dead remains and waste products through
decomposition.
Microbes also have an important place in most higher-order
multicellular organisms as symbionts. Many blame the
failure of Biosphere 2
on an improper balance of microbes.

Use in food

They are also used to control the fermentation
process in the production of cultured dairy
products such as yogurt and cheese. The cultures also provide
flavour and aroma, and inhibit undesirable organisms.

Use in water treatment

Microbes are used in the biological treatment of
sewage and industrial waste effluents..

Use in warfare

In the Middle Ages, dead corpses were thrown
over walls during sieges, this meant that any bacteria carrying the
disease that killed the person/creature would multiply in the
vicinity of the opposing side.

Importance in human health

Human digestion

Microorganisms can form an endosymbiotic
relationship with other, larger organisms. For example, the
bacteria that live within the human digestive system contribute to
gut immunity, synthesise vitamins such as folic acid and
biotin, and ferment
complex indigestible carbohydrates.

Hygiene

Hygiene is the avoidance of infection or food spoiling by eliminating
microorganisms from the surroundings. As microorganisms,
particularly bacteria,
are found practically everywhere, this means in most cases the
reduction of harmful microorganisms to acceptable levels. However,
in some cases it is required that an object or substance be
completely sterile, i.e. devoid of all living entities and viruses. A good example of this is
a hypodermic
needle.

In food preparation microorganisms are reduced by
preservation methods (such as the addition of vinegar), clean utensils used in
preparation, short storage periods or by cool temperatures. If
complete sterility is needed, the two most common methods are
irradiation and the
use of an autoclave,
which resembles a pressure
cooker.

There are several methods for investigating the
level of hygiene in a sample of food, drinking water, equipment
etc. Water samples can be filtrated through an extremely fine
filter. This filter is then placed in a nutrient
medium. Microorganisms on the filter then grow to form a
visible colony. Harmful microorganisms can be detected in food by
placing a sample in a nutrient
broth designed to enrich the organisms in question. Various
methods, such as selective
media or PCR, can then be used
for detection. The hygiene of hard surfaces, such as cooking pots,
can be tested by touching them with a solid piece of nutrient
medium and then allowing the microorganisms to grow on
it.

There are no conditions where all microorganisms
would grow, and therefore often several different methods are
needed. For example, a food sample might be analyzed on three
different nutrient
mediums designed to indicate the presence of "total" bacteria (conditions where
many, but not all, bacteria grow), molds (conditions where the growth
of bacteria is
prevented by e.g. antibiotics) and coliformbacteria (these
indicate a sewage contamination).

In fiction

Microorganisms have frequently played an
important part in science
fiction, both as agents of disease, and as entities in their
own right.